A conventional hydraulic brake booster operates in three different modes namely rest mode, apply mode and hold mode. In conventional booster assembly during rest mode boost cavity is connected to outlet port and pump port is directly connected to steering port and no pressurized fluid is supplied to boost cavity and is maintained at atmospheric pressure. The boost cavity is directly connected to the outlet port and accumulator is charged on steering application. During apply mode i.e. when the pedal is operated by the driver the input rod moves the spool valve to close the reservoir port and the steering port. The pump port opens into the boost cavity and pressurized fluid is supplied to boost cavity. The hydraulic pressure generated behind a piston is used to generate boost force. During hold mode i.e. when the driver retains the pedal in same position the boost piston is moved towards left and lever is rotated about the eccentric sleeve and as a result spool valve moves towards right and the ports to boost cavity are closed.
In the hydraulic boosters that are hitherto available in the market, the high-pressure fluid comes from the steering circuit. Whenever high pressure is required for the operation of the brake booster, the steering circuit is closed, thereby getting enough high-pressure fluid to the brake circuit. This has the drawback of possible steering loss during brake applications. Secondly, in some of the currently available hydraulic boosters, because of the high ratio of input to output travel, there is a need for arresting the extra travel during manual operations. This requires a special complicated travel limiter assembly. Thirdly, in the currently available hydraulic boosters, the input rod and the trigger mechanism are in two different axes, requiring complicated trigger mechanisms. Fourthly, in the currently available hydraulic boosters, the input-output relationship depends on the input ramp rate. The performance of these boosters is therefore ramp rate sensitive. Also, the currently available boosters tend to give less output at high ramp rates compared to lower ramp rates. This has the drawback of brake not being effective at high ramp rates. It is indeed desirable, considering the wide operating range of the vehicle, to have a booster that gives consistent performance. Fifthly, in the currently available hydraulic boosters, the accumulator comes into force only during fail-safe mode. The trigger mechanism for the fail-safe mode is quite complicated and involves very intricate machining.
There exists a need for a hydraulic brake booster capable of overcoming these technical aspects to maximize output pressure and enhance overall performance.